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Coordinates.md

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Coordinate systems in Diablo2

Diablo2 has quite a few coordinate systems with different precisions. We'll try to list them with their various precisions using terms that will be reused in the code to facilitate communication and sharing. This list is non-exhaustive.

D2 Coordinates

Coordinates

The game uses a 2d grid with various resolutions depending on what it needs to achieve. This 2d grid is then projected using an Isometric projection (actually a Dimetric projection which leads to a 2:1 ratio).

To avoid simplify things and avoid naming issues, we'll be speaking of "Game coordinates" for all the logic, and "Client" coordinates for the projected coordinates.

Game coordinates

To simplify, game coordinates are aligned with the tiles axes.

The following formulae can be used to transform between game and client coordinates (assuming the same precision).

Game to client (dimetric projection)

  • clientX = (gameX - gameY) / 2
  • clientY = (gameX + gameY) / 4

Client to game

  • gameX = 2 * clientY + clientX
  • gameY = 2 * clientY - clientX

Precision of the coordinates

Precision Ratio to parent Size in game coords Names in D2Moo Desc
Room 1 40 RoomX/Y Used to group units and tiles in a single entity
Floor tiles 8 5 TilesX/Y Group of subtiles used for map edition
Subtiles/Gamecoords 5 1 X/Y Used to determine floor collision flags and properties. (Called GameSquare in the original code?)
Fractional coordinates 2¹⁶ = 65536 1/65536 PrecisionX/Y Used for paths, 16bit fixed point representation of the Game Coordinates

There are also Light coordinates: 1/8 subtile (to verify).

Cardinal points

Some code/assets may refer to cardinal points for orientation (for example the Rogue Encampment exit).

  • East: Positive X
  • West: Negative X
  • South: Positive Y
  • North: Negative Y

Client coordinates

The client coordinates are aligned with the screen axes (but not the same oritentation!). They are the coordinates obtained by going through the dimetric projection. This matches the pixels representation of the game (if we ignore the fake perspective option). All graphics assets are represented in this coordinate system.

A "square" of the Game coordinates grid is represented as a diamond shape in the client coordinates. Note that walls are however represented in blocks of 32x32 pixels.

Precision Ratio to parent Size in pixels
Room 1 1280x640
Floor tiles 8 160x80
Subtiles/Gamecoords 5 32x16
Wall block 1/2 32x32

Dimetric projection

Note that in the (Static|Dynamic)PathStrc structures, both Game and Client coordinates are stored. The Client coordinates precision is in pixels, and the Game coordinates are stored as Subtiles for static paths, and Fractional for dynamic paths.

Conversion functions

Coordinates A Coordinates B A => B 1.10f ordinal B => A 1.10f ordinal
Game tile Client pixel DUNGEON_GameTileToClientCoords D2Common@10110 DUNGEON_ClientToGameTileCoords D2Common@10107
Game subtile Client pixel DUNGEON_GameSubTileToClientCoords D2Common@10111 DUNGEON_ClientToGameSubtileCoords D2Common@10108
Game unit Client unit DUNGEON_GameToClientCoords D2Common@10112 DUNGEON_ClientToGameCoords D2Common@10109
Game Tile Game Subtile DUNGEON_GameTileToSubtileCoords D2Common@10113 N/A N/A
Game subtile Fractional PATH_ToFP16Corner/PATH_ToFP16Center N/A PATH_FromFP16 N/A
Fractional Client pixel PATH_UpdateClientCoords N/A N/A N/A
Game tile Client (Render position) DUNGEON_GameToClientTileDrawPositionCoords* D2Common@10115 DUNGEON_ClientTileDrawPositionToGameCoords* D2Common@10114
Game subtile Client (Render position) DUNGEON_GameToClientSubtileDrawPositionCoords* D2Common@10117 DUNGEON_ClientSubileDrawPositionToGameCoords* D2Common@10116

*The Client(Sub)TileRenderPosition functions will provide the draw position in Client space of the source unit.

A picture is worth a thousand words. Given a Tile/Subtile (green) game coordinate (X;Y), you will obtain:

  • The BLUE dot when using the Game(Sub)TileToClient(Sub)TileRenderPosition functions. While this is the rendering position, this is not in screen coordinates, Y is still facing down!
  • The RED dot when using the Game(Sub)TileToClient functions.

Render position conversion

Screen position

This is the coordinate system used for the GUI. Starts at the bottom-left of the screen. X faces to the right, Y faces up.

Naming convention in D2Moo

TODO